Reproduction of a continuous-tone image on a halftone printing device, such as an offset press, electrographic printer, inkjet printer, or the like, involves conversion of the continuous-tone image to bi-level data. The conversion process, commonly called halftoning, produces halftone dot patterns that represent continuous-tone imagery when integrated by the human eye. The bi-level data takes the form of a set of color separation bitmaps. Each bitmap corresponds to one of the color channels used by the halftone printing device, e.g., cyan, magenta, yellow, and black (CMYK). The bitmap for each color channel specifies the presence or absence of colorant at a given location on the printed page. In other words, the halftone dot pattern defined by the bitmap specifies the placement of colorant dots within the printed page for the respective color channel.
In general, halftoning techniques fall into one of two categories: dispersed dot or clustered dot. Clustered dot halftoning methods tend to increase the size of one or a small number of ink spots within a given area as the corresponding continuous-tone pixel decreases in lightness. Dispersed dot halftoning techniques, on the other hand, use nearly constant diameter dots and increase the number of dots present in a given area as the continuous-tone pixel lightness decreases. In either case, the objective is generation of a halftone cell containing dots that, when integrated by the human eye, approximate the appearance of the original pixel in the continuous-tone image to be reproduced.
Dot placement according to dispersed dot methods may be either periodic, such as with Bayer ordered dither, or pseudo-random, as with error diffusion and stochastic screening. Stochastic halftone patterns can be implemented using precomputed arrays to avoid extensive calculations in the page rendering process. The arrays can be thought of as masks that are applied to continuous-tone values on a pixel-by-pixel basis for individual color channels to produce halftone dot patterns.
One form of precomputed array is a threshold array. A threshold array, for M=2N levels, is a two-dimensional array with N-bit integer entries. An alternate form of precomputed array is a halftone “volume,” which is a set of M two-dimensional bitmaps corresponding to the M halftone levels. A set of halftone volume bitmaps may be generated from a threshold array. Images reproduced using either a threshold array or halftone volume may be substantially identical in content. In each case, the distribution of dots for each color separation depends on the characteristics of the precomputed threshold array mask.